Technical Field
The present disclosure generally relates to the use of fully depleted silicon-on-insulator (FD-SOI) transistors as substitutes for passive capacitors, and to applications of such capacitive transistor structures in improving performance of analog microcircuits.
Description of the Related Art
Analog microcircuits and, in particular, integrated analog circuits built on semiconductor substrates, are used widely in medical electronics, aerospace systems, and devices equipped with RF communications capability. Such RF-equipped devices include smart phones, tablet computers, microelectronic sensors, and the like. Specific examples of analog microcircuits include reference voltage generator circuits and phase locked loops. Reference voltage generator circuits are used to produce a constant reference voltage in voltage-regulated devices such as power supplies and analog-to-digital converters. Phase locked loop (PLL) circuits are used to perform phase matching between input and output signals of oscillators. Such circuits can be used, for example, in microprocessors for clock synchronization, and in telecommunications for signal demodulation and frequency synthesis.
Capacitors are frequently used as components in analog microcircuits. Various structures that are present on integrated circuit chips can be made available to circuit designers for use as parallel plate capacitors, or such capacitors can be specially designed. Alternatively, it is possible for metal-oxide-semiconductor field effect transistors (MOSFETs), which are ubiquitous on integrated circuit chips, to be used as capacitors in various microcircuits if the components are connected in a particular manner. In a field effect transistor (FET), a thin gate oxide layer between the gate and the channel can be considered a capacitive dielectric, and the gate and the channel regions can be considered two parallel plates of a capacitor. However, because typical gate oxide layers are only a few nm thick, the gate oxide layer tends to be vulnerable and can break down easily in response to application of a gate voltage that is slightly too large. Furthermore, transistors are active, three-terminal devices whereas capacitors are passive, two-terminal devices that are generally much simpler and less expensive to manufacture. For at least these reasons, it is generally not customary to connect transistors in such a way that they can be used as capacitors.